This invention relates to papermaking and more particularly to a method and apparatus for reducing streaking in a paper web during its manufacture. In the papermaking field, the term "streaking" refers to non-uniformity of a paper web across its width, the latter often referred to as the cross direction and denoted as CD. One of the most troublesome CD streaking effects is due to variations of moisture content.
Uniformity of moisture, caliper, smoothness and weight profiles across the width of a paper web are critical to efficient production. In many instances, the product is treated or processed in some manner to a greater and undesirable degree than ideal, as a simple result of cross-machine direction (CD) moisture profile variability.
A familiar example is over-drying the web in the main dryer section of a paper machine to a very low moisture content at the size press in order to reduce the magnitude of moisture peaks that would otherwise occur. Such moisture peaks may originate in the forming process, press section or dryer section for any number of reasons. Over-drying, i.e., drying product to an average moisture content below what is considered ideal, can be accomplished by operating the machine at a slower speed, which significantly reduces productivity and value of the operation. Alternatively, over-drying can be accomplished by adding additional heat to the web during its drying stage, which adds considerably to the manufacturing costs.
The low productivity and/or increased energy costs discussed above are consequences of dealing with excessive CD moisture variation. In addition, moisture variability has a negative impact on product quality. For example, the development of web compaction, wet and dry web strength, sizing efficiency, surface smoothness and stress concentrations in the web are all functions of web moisture, thereby producing similar variability in web properties across the width of the web. Eliminating or reducing the moisture variability early in the papermaking process would be extremely valuable to the paper industry.
While these methods for reducing moisture variability are in common use throughout the industry, they can be considered as treating the symptoms and not the root cause of the problem. In many cases, the root cause is moisture streaks originating in the press section. Such streaks may result from roll cover wear profiles, shower pluggage, or non-uniform development of felt compaction, felt void volume, web release properties, and the like. In addition to streaking due to unevenness in the apparatus and elements which contact the paper web during its manufacture, it may also be caused by naturally occurring zones of unevenness of caliper across the web width, or other naturally occurring anisotropies in the paper, as the web is progressively formed. As an example, if the paper web is formed with one or more width-wise zones thicker than all of the remaining width-wise zones of the web, then upon the web passing through the nip of two rolls, those thicker zones will be squeezed more than all of the other zones, and hence will exhibit different moisture contents. Hence even if the nip of two given rolls were to exert a perfectly uniform pressure on a web of perfectly uniform caliper (thickness), those same two rolls squeezing a paper web of non-uniform thickness, weight, or moisture content would yield a web of non- uniform moisture, thickness, smoothness, or other web property in the cross direction (CD), and streaking would result.
Existing methods for improving moisture uniformity in the press section include crowning of press roll covers, use of profiling steam boxes (or profiling infrared energy), control-crowned rolls and various conditioning treatments to the press section clothing (localized chemical or mechanical cleaning of press felts). All of these have proven useful to various degrees, but have severe limitations. Crowning of the press roll covers, for example, is useful for generating a level nip loading across the width of the machine, but cannot be used to influence moisture at a certain CD location. In addition, a fixed cover crown is appropriate for only one loading level and cannot be changed without removing the roll from the machine. Controlled-crown rolls have been used for many years with some success and can be useful for altering nip load distribution, but only in a very broad sense. This is because control crown rolls operate by applying internal pressure to the metal roll shell, which is quite thick and necessarily very stiff. They are not useful for addressing local moisture problems, since CD nip loading cannot be controlled over short distances.
The most effective moisture profiling strategy in the press section is probably the use of steam boxes or infrared preheat ahead of the press nip. These devices facilitate nip dewatering by lowering the viscosity of water in the web. Different amounts of sensible heat are added to the wet web across its width, in a manner corresponding with measured CD moisture profiles at a downstream location. In general, profiling steam boxes have been successful, as measured by elevated size press moistures, increased machine speeds and reduced CD moisture variability.
The main shortcomings of steam box use are three. First is the increase in web density as the web is pressed at a higher temperature. Densification in the nip increases with increasing temperature, making this an unacceptable strategy for many bulk-sensitive grades. Second is the practical limitation on CD control width segments in the steam box. Typically, one to four foot wide control zones are employed. In many instances, it would be desirable to obtain a finer degree of CD control. Thirdly, steam boxes positioned above the web are prone to drip condensate onto the moving web, thereby compromising product quality.
As used herein, the term roll cover refers to a nonmetallic covering, 0.1 to 2.0 inches thick, of a cylindrical shaft or solid cylinder. Common roll cover materials include, but are not limited to, rubber and polyurethane compounds and variations thereof.
Roll covers can be characterized by their modulus of elasticity, which is a ratio of incremental stress to the resulting incremental strain. This property is dependent on the composition of the material in question, among other things. Changes in the modulus of elasticity of a roll cover affect the shape and magnitude of machine direction pressure profile in a rolling nip. The term effective modulus as used in this invention is intended to designate any cover property which, when changed, can similarly affect the pressure profile in a rolling nip. Changes in cover density, thickness, firmness, hardness, compressibility, position relative to the roll shaft, and the like are hereby referred to as equivalent to changes in the effective modulus of the cover.
Previous attempts to control the stiffness, hardness, or resiliency of a roll cover which are related to the present invention can be found in U.S. Pat. Nos. 4,928,593 (Ruckl) and 5,634,606 (Roder). These inventions rely on an inflatable elastic bag ('593) or annular gas filled cells ('606), located under the subject roll cover, which can be pressurized to various levels. In the third embodiment of Roder's press-on roller ('606), a plurality of annular gas-filled cells spaced along the width of the roll, each cell equipped with a dedicated pressure valve, is proposed for CD profiling of the nip pressure. However, the press-on roller in '606 would not be suitable for use in the paper machine.
The present invention overcomes the limitations of existing technologies for improving CD moisture uniformity in the press section. The basis for this invention is that intrinsic, or effective, properties in the roll cover itself are controlled in a fashion that makes possible much higher CD control resolution. Further, the present invention provides means for globally changing the hardness of the roll cover across its entire width, thereby allowing different cover hardnesses to be used as situations dictate. For example, in some press positions, softer roll covers may be better suited for heavier weight grades of paper in order to extend nip dwell time, while harder covers may be preferred for lighter weight grades which require less dwell time but greater nip pressure. Changing cover properties across the full roll width also provides the papermaker a tool with which to eliminate certain quality or production problems, including poor water removal efficiency, crushing in the first nip, felt marking, excessive densification on bulk sensitive grades, and the like.
In the present invention, roll cover properties are altered and controlled in a manner such that the shape and magnitude of the pressure pulse in a nip formed by two rolls, a roll and a blade, or a roll and a rod, is controlled to a desired level. This is accomplished by incorporations an electorheological or magnetorheological elastomer into the roll cover and using an electric field or a magnetic field to stimulate the elastomer such that its physical properties are effectively changed.
As used herein, the term electrorheological elastomer refers to any of various elastomeric compounds, including but not limited to rubber and polyurethane, which contain or are in close proximity to polarizable particles which can be acted upon by an external electric field. The particles can be located in any of a variety of configurations relative to the elastomeric compound. For example, discrete droplets of electrorheological fluid (ERF) can be dispersed throughout the cover material during the cover's manufacture, such that there will always be a reserve of active particles during the life of the roll cover. The cover, which serves as a carrier for the particles, should be non-conductive. As another example, one may suspend the polarizable particles not in viscous fluid pockets or droplets, but rather in the form of a viscoelastic solid or electrorheological gel (ERG) as taught in U.S. Pat. Nos. 5,607,996 (Nichols et al) and 5,364,565 (Li et al). Such material is described as an electrorheological solid (ERS). Viscoelastic properties such as loss factor and shear modulus for these materials are controlled by the electric field used.
Similarly, the term magnetorheological elastomer is used as the magnetic analog of the electrorheological elastomer discussed above. While several methods exist for introducing the magnetizable particles in or close to the roll cover elastomer, the method discussed by Watson (U.S. Pat. No. 5,609,353) which relates to a magnetoviscoelastic solid is of special interest.
The present invention is not limited to press section roll covers for improved CD moisture profile. The concept can be applied to size press and coater backing rolls for more uniform coat-weight application and coating distribution across the web thickness. It can also be applied to the covered rolls in virtually any type of calender (gloss, hot-soft, super, extended nip, etc.), depending on temperature limitations. The invention is useful in improving smoothness, gloss, and caliper profiles when used in a calendering process. One method of altering the intrinsic or effective roll cover properties over short distances involves the use of electrorheological (ER) materials or magnetorheological (MR) materials imbedded within the cover or directly under the cover.
Numerous technical articles on electrorheological (ER) fluids have been written. For example, the Journal of Rheology, Vol. 42, issue 3, May/June 1998, contains the following articles: (1) Kanu, R., and Shaw, M., "Enhanced Electrorheological Fluids Using Anisotropic Particles," pp. 657-670; (2) Rankin, P., and Klingenberg, D., "The Electrorheology of Brium Titanate Suspensions," pp. 639-656; and (3) Yao, N., and Jamieson, A., "Electrorheological Creep Response of Tumbling Nematics," pp. 603-619. Each of these articles cites numerous references themselves. Potential uses of ER fluids, as mentioned by Rankin and Klingenberg, include shock absorbers, engine mounts, clutches, brakes, artificial joints, among others.
One example of the use of an electrorheological (ER) fluid for changing the effective hardness of a hollow elastomer member is shown in U.S. Pat. No. 5,390,974 issued to Theodorakakos. A number of plates are positioned within the hollow member and when the plates are charged, the stiffness of the elastomer increases. A similar result is achieved, using an elastomer impregnated with a fine powder, the powder polarizing when subjected to an electric field and changing the viscoelasticity of the elastomer, by the disclosure of U.S. Pat. No. 5,290,821 issued to Sakurai et al. Another example of the use of an electric field to change the stiffness of an elastomer containing polarizable particles is shown in U.S. Pat. No. 5,607,996 issued to Nichols et al. U.S. Pat. No. 5,609,353 issued to Watson discloses an elastomer suspension for an automobile bushing, the bushing having embedded therein a plurality of iron particles, the latter suspended in a carrier of low magntic permeability. The magnetic field from a contiguous coil acts upon the iron particles to thus vary the elastomer stiffness. Hence the worker in this art may use either embedded polarizable particles or embedded magnetic particles, to vary the stiffness of an elastomer member. Each of the above literature references and patents is hereby incorporated by reference.